Karyotypes are described for Micruroides euryxanthus from Arizona and Micrurus tener from Texas. These are compared with karyotypes of other elapids from around the world, which exhibit significant interspecific variation. The largest macrochromosome of M. euryxanthus, which is metacentric, is shared by only two other species of coralsnakes from the New World. This may be a shared ancestral chromosome homologous to the largest macrochromosome that occurs in most other snakes, including some of the Australian elapids. The karyotype of M. tener from Texas has a ZZ:ZW₁W₂ sex chromosome system, which differs from individuals of this species reported previously from Louisiana. Over the relatively young 35-million-year global history of the Elapidae, karyotypes appear to have varied more than those of most other snakes throughout a 140-million-year history.